Method of geophysical prospecting



Feb. 13, 1940.. PETERSON 2,190,324

METHOD OF GEOPHYSICAL PROSPECTING Filed Feb. 2, 1938 4 Sheets-Sheet 1 EMFar 0016 9072; 3

' INVENTOR fin fieferqon BY M1 ATTORNEY.

Feb. 13, 1940. ca. PETERSON METHOD OF GEOPHYSICAL PROSPECTING Filed Feb.2, 193B 4 Sheets-Sheet 2 IN VEN TOR. 676/7 e7erson,

ATTORNEY.

G. PETERSON METHOD OF GEOPHYSICAL PROSPECTING Feb. 13, 1940.

Filed Feb. 2, 1938 4 Sheets- Sheet 3 INVEN TOR. 6/90 e7er.so/2,

ATTORNEY.

Feb. 13, 1940. a. PETERSON METHOD OF GEOPHY$ICAL PROSPECT-ING Filed Feb.2, 1938 4 Sheets-Sheet 4 INVENTOK, W 6/6 Paris/Jon,

ATTORNEY.

' lo]. its-1st) invention'relates to geophysical prospecting andparticularly to a method of, and apparatus for, geophysical prospectinginvolving measurements which depends-upon the electrolytic polarisationcharacteristics .of-imderground' formations. I

An object of -my invention is to' provide a method of geophysicalprospecting bywhich underground formations and structures may be 10indicated by measurements which reflect the de- A gree of electrolyticpolarization of the earth and the manner in which the polarizationbuilds up and discharges. g Y

Another object is to provide apparatus by 1| which the-aiore-mentionedmeasurements may conveniently be made. I

These and other apparent objects I attain in a manner which will beclear from a consideration, of the following description taken inconmnection with which:

Fig. l is a diagrammatic illustration of a simple arrangement todemonstrate the underlying principles ofmy invention.

a Fig. 2 is a'diagram showing the manner in which the polarizingcurrentin the circuit of Fig. 1 changes with time. w

Fig. 3is a diagram showing the manner in I which the E. M. F. ofpolarization of the earth varies with time.

Fig. 4 is a diagrammatic illustration of a circuit arrangement employedin the practice of my invention. v v

Figs. 5, 8, and '1 are diagrams illustrating the variation of E. M. F.of polarization. with time under three different conditions employed inthe the accompanying drawings, of

practice of mymethod, and illustrating the rela- '9 alternativecircuitarrangement that may be em-" ployed in the practice of myinvention.

Fig. 14 is a diagram-illustrating the type of 4 curves derived from dataobtainable by use of my method. showing the variationof thepotentialdiflerenceduetoearthpolarisationduring the build-up and thedecay "of polarization for various polarizingelectromotive forces actingover one particular polarizing time period.

Fig. 15 is ajdiagram illustrating the variation of the maximum potentialdifference due to'earth 5 polarization, obtained during the build-up anddecay of polarization. with a change in the distance between electrodes,for two different polarizing electromotive forces.

My invention is based upon the phenomenon of'electrolytic polarizationwhich can best be understood by reference to Figs. .1, 2, and 3.Consider the simple circuit consisting of two spaced.

- electrodes I0 and ii in electric contact with an electrolyte i2, andin circuit with a source l3 of direct current, and a switch ll.v Whenthe switch His closed, current starts to flow through the circuit withan initial value Imax. determined by Ohms law, but gradually decreasesto a minimum Imin. due to polarization which is built up in theelectrolyte. In the*case of most electrolytes the decrease of currentwith timeis about as represented in Fig. 2. In the case of oil, however,there is a delay of about 1 second before the current is observed todecreasebypolarization.

The polarization which is responsible for the decrease of current in anelectrolytic circuit may be considered as mainly due to theconcentration of electrically-charged ions at inhomogeneities in theelectrolyte, amounting in eifect to an electromotive force in theelectrolyte portion of the circuit in opposition to the applied E. M. F.

Thus, in the apparatus of Fig. 1, negatively charged ions are ioundinthe neighborhood of positive electrode Ill and positively-charged ionsare-found in the neighborhood of negative electrode ll.

Theearth conducts current chiefly as an electrolyte, and since it iscomposedof many subin strata and lenticular deposits, there are manyinhomogeneities and many obstacles to the free passage of ionstherethrough. The various boundaries between dissimilar formationsexisting in the earth, therefore, afiord excellent opportunity for theaccumulation of ions and the resultant production of polarizationeffects.

' Since the polarization efiects are dependent upon the undergroundformations and structures in which they arise, they may be used for thedeter- '50 mination of these formations and structures.

Assuming the switch it is closed at time To. the

E. M. F. of polarization, corresponding to decrease of current in thecircuit, will build up according to the curve of e shown from time To tou to recognize underground formations.

Ti. It will be observed that'as the current decreases asymptotically to1min. the- E. M. I". of

polarization increases and asymptotically approaches the value em. Therelations existing between the various quantities are as follows:

circuit; R is the ohmic resistance of the electrolyte between theelectrodes.

At time T1 if the switch H is opened, the

E. M. F. of polarization e will discharge itself through the electrolytebetween the-concentrations of oppositely-charged ions and will, inconsequence, decrease rapidly and asymptotically approach zero, which issubstantially reached" at time T2. When the switch I is openandpolarization is decreasing, a potential difference, proportional tothe E. M. F. of polarization-exists between the electrodes l0 and II,and the manner in which'the E. M. F. of polarization e decreases to zerois reflected by the manner in whichv the potential difierence at theelectrodes decreases. The extent and shape of the curveof Fig. 3 isimportant and is dependent upon the character of theelectrolytically-conducting material traversed .by the polarizingcurrent. Thus, when a region of the earthis polarized by application ofan electromotive force thereto, the character of underground formationsand structures may be determined from analysis of the degree ofpolarization and the manner in which it builds up and discharges.

The degree of polarization for the same applied E. M. F. is differentfor different substances,

and this effect alone, as indicated by differencein the value of emax.or of 1min. maybe employed Oil-bearing sand is prominent in exhibiting adifferent degree of polarization from other substances which maysurround it.

The presence of oilin a. formation also tends to delay polarization by atime interval which in pure oil is of the order of 1 second. The shapeof the curve of E. M. F. of polarization vs. time for earth containingoil, therefore, should be different from that for ordinary electrolytes.The energy absorbed by the electrolytic process is also importantbecause it is characteristic of the particular earth formations throughwhich the current passes, and when the energy or a function of theenergy is determined, it may be used to indicate the composition ofunderground formations. The theoretical value of energy absorbed is T (4W: E f IdT T being time and having the same subscripts as in Figs. 2 and3.

According to my invention, I provide a method of and apparatusv forexamining the polarization effects of the earth and for finding a curvewhich is related to the curve of E. M. F. of polarization vs. time,shown in Fig. 3, as will hereinafter be described in detail. The curvewhich I can determine by my method is significant because the curve ofFig. 3 is significant, and it will be obvious, therefore, that theinformation contained in the curve which my method enables me todetermine constitutes a valuable tool in geophysical prospecting to aidin determiningboth the structure mations. a I v 'I prefer to 'determinea curve. related to that of Fig. 3 in the following manner. The circuitdiagrammatically illustrated-in Fig. 4 is prefer- .erablyemployed'.Spaced non-'polarizingelecand compositional underground'fortrodes II andII are electrically connected to of relay l'l, which, .whenrelay switchI2 is closed, connects the electrode ll through switch 22 to thepositive terminal oLa source I! of direct current, the negative terminalof which is'connected to the electrode it through a direct .Lcur: rentmeasuring instrument 2I. When the switch It is opened and switch '2 I isclosed, the electrodes it and 16 areconnected through the measuringcircuit comprising direct current indicating instrument22 and thebalancing circuits- 22 and 24 in series. The balancing circuit 24comprises a double-pole, 'doublethr'ow reversing switch 2! with abattery 28 connected to two oii'the' stationary terminals and apotentiomfearth I2. Electrode ll is connected 'to an'arrnstar 21connected to the movable terminala'the other stationary terminalsbeingcross connected with the to cause a-potential diflerence of Y acertain value to be applied to potentiometer 21 in one position of theswitch and an equal but opposite potential difference to be appliedthereto in the other position of the switch. The bal-' ancing circuit 22comprises the double-pole, double-throw reversing switch 28, the battery22, and the potentiometer ll connected in the same manner as in thecircuit 24; and in addition has the voltage measuring instrument 3|connected across the potentiometer 30. Y Y

The relay l1, detailed description of one .form

of which will hereinafterLbe-given, is adapted to close switch II andremain closed for a predetermined time interval, then open and remainopen for another relatively predetermined time interval, then closeswitch 2| and remain closed for another predetermined interval, thenopen and repeat the cycle. During the time when switch "is closed,polarization is being built up in the Y i '45 earth andthe E. M. F. ofpolarizationis increasing in a manner similar to that part of the curveof'Fig; 3 between To and Ti; while during the time when switch Ibisopen, the earth polarization is being discharged and the E. M. F. ofpolarization is decreasing in a manner similar to that part of the curveof Fig. 3 between T1 and T2. During the time when switch 2! is closedthe potential difierence between the electrodes l5 and it due to earthpolarization previously/ set up is impressed upon the measuring circuit.

In making observations, I first close the relay switch 2|, the switches25 and 28 being open;

Ordinarily a small earth current will be observed to pass throughinstrument 22. Then with the potentiometer 21 set to insert zero E. M.F. in the measuring circuit, I close the switch 25 in a direction tocause a potential dlf- I ference across potentiometer 21 which is ofproper direction to neutralize the current flowing through instrument22, and I then gradual.- ly adjust the potentiometer 21 until no currentpasses through the instrument 22., With the normal earth currentsbalanced out in this-manner, I then proceed to repeatedly polarize theearth and measure potential difierences between the electrodesduring'various intervals of time in a manner that can best be understoodby reference to Figs. 5, 6,and '7. Y

With switch as closed, I may close the switch I8 at time in and allow itto remain closed until earth polarization starts to v At" time after aninterval t=b1ci has elapsed the switch 2| a closed connecting.thevoltage measuring circuit to measure the potentialdifference-betw'een the electrodes during the time interval .cidi. At'time drthe switch "is opened and at time a: the cycle is repeated. thetime intervals for each of the foregoing periods in each cycle I ireturned to zero and therefore starts to build up from a value higherthan in the first cycle. Hence, atthe end be of the polarization periodof the second cycle. the E. M. F. of polarization has reached a valuehigher than in the first cytraced by the dottedline L.

The instruments 2' and 22 are preferably of a type that will indicatethe average current pass 'ing through them over the longest cycle thatis to be used. Thus. the instrument 2! measures the average polarizingcurrent throughout thev cycle and the instrument 22 will be affected bythe average potential difference between the electrodes due to the E. M.F. of polarization during the measuring period c1d1, again averaged overthe cycle. By knowing the average throughout the cycle and what portionof the cycle the currents are active it is possible to compute theaverage-value of polarizing current and electrode potential differenceduring the time periods when they are active.

In measuring the electrode potential difference, I may make themeasurement directly on the instrument 22 as described above, but I.preferably use the instrument 22 only as an indicating instrument andadjust the potential diiference of potentiometer 30 to include in serieswith instrument 22 a suflioient potential difference to just neutralizethe potential difference due to E. M. F. of polarization. The potentialdifference as determined fron'rthe voltage read on instrument II and theposition oif'- but with successively longer intervals 1. between thetermination of the polarizing period and the beginningof the measuringperiod, and therefore successively longer cycles. It will be observedthat in Fig. 6 the period t is longer than in Fig. 5 and hence thepotential difference is measured further along the polarization decaycurve. One result of this is that the polarizing period of the secondcycle in Fig. 6 does not start with so high an E. M. F. as in Fig. 5;and therefore in Fig. 6 there is not such a great diiierence between theequilibrium value of maximum E. M. F. and that which exists after thepolarizing period of the first cycle. In Fig. '7 the time period t andthe cycle length are still longer than in Fig. 6, and the potentialdiflerence is measured at a point still further down on the polarizationdecay curve. Also there is very little difference between theequilibrium value of-the maximum amass) time. Inf when theswitch-flisopened the II. 'M. F. andthe after the wiarizingpor odoi'theflrltwcle. aI

' I'may now plot the average value of electrode potential diiierence. F,G, and H. during the measuring of Figa'ii, (Land '1, asin Fig.

8 against the corresponding times and a curve may be drawn through them,as indicated. In

' practice, of course, I make" many more observations corresponding'todifferent intervalst. in

order to be able to plot ,the curve accurately'all along its length.While this curve is'not exactly that .of-"polarization E. M. F. vs.time'because of the fact that the potential diflerences measured areaverages over a substantial time period and because of the diiierenceeinequilibrium value oithe maximum polarimtion E. M. F. under theconditionsof the various measurements; nevertheless the curve representing decayofpolarlzation. in Fig. 8 is dependentljupon andrelated to the truecurve as shown in Fig. 3,

' charging curve of Fig. 3, I may make. afseries of measurements underthe conditions illustrated in Figs. 9, 10,'and 11, in which the variousperiods'of the cycle are designated similarly to those of Figs. 5, e,and '1. Instead of sholding the polarizing period ab constant, it issuccessively increased 'in this series, and the measuring period ed, thetime interval be, and the total time oi one cycle, are held constant.The measuring period is in each case taken as close to the.

end of the. polarizing period as is convenierrt.

When equilibrium is attained, the average potential diflerences K, M,and N, existing during the measuring period aremeasured, and may then beplotted against the corresponding time .as in Fig. 8 to represent thepoints on the charging curve, from To to T1. While there is an error in'4 assigning these values of potential diflerence measured on thedischarge curve, as points on the charging curve, the error may bereduced'by making the measuring period closely follow the termination ofthe polarizing period. In any case, thecharging curve plotted from aseries of measurements as K, Myand N is related to .the actual chargingcurve of Fig. 3, andmay rate of change is the conductance of which the Yresistance is the reciprocal. The value'of' Cmax. found in this way maybe assigned to the maximum in the curve of Fig. 8, and correspondingvalues of 2 may then be found for various times T from the curve of Fig.8. By use of Equation 2, values of I may then be found for these valuesof T and maybe employed in Equation 4 to get a value which, while it isnot strictly the energy absorbed in the polarization process. is relatedto the energy and may be employed as indicative of undergroundformations traversed by the polarizing current.

While, in the foregoing procedures, the polariz; ing electromotive forcehas been assumed constant, I may vary it, holding the polarizing time abconstant and measuring the electrode posive'ly increased by equalamounts, the

tential difference immediately following the termination of thepolarizing period in the case of each "particular value of polarizing E.M. F.; and I may make the entire series of tests previously-describedwith each of a plurality of different polarizing electromotive forces.An important result of conducting the tests with vari-,

nus-values of polarizing E. M. F. is that with larger values of,polarizing E. M. F. the earth layers near the surface become saturatedwith respect to polarization and increases in polarization are mainlycontributed by the deeper earth layers, the composition of which it isdesired to Thus, as illustrated in Fig. 14,-'I-mayobtain the curve ofpotential difference vs. time during the build-up of polarization over apolarizing period ab and for an applied polarizing E. M. F. El aspreviously described in connection with Figs. 9, 10, and llfand may thendetermine the curve of potential difference vs. time during thedischarge of polarization as previously described in connection withFigs. 5, 6, and 7, plotting the complete curve E1 in Fig. 14, as was Idone in Fig. 8. The charging curve over the period ab is then determinedin a like manner for a greater polarizing E. M.- F. E2, thecorresponding discharge curve is determined and the complete curve isplotted as curve E2 in Fig. 14. In.

a similar manner curves E3 and E4 corresponding to progressively greatervalues of polarizing E. M. F. are determined and plotted in Fig. 14 ascurves E3 and E4 respectively.

As shown in Fig. 14, the curve E1 in the time interval ab is nearlystraight because the applied E. M. F. is so small that not even theearth layers near the surface are saturated in the sense of reachingtheir highest possible polarization. When the polarizing'E. M. F. isincreased to E2 the charging curve is more curved, showing more tendencyto approach an upper'limit, and the potential difference P2 due topolarization at time b is considerably greater than that of P1 when theapplied E. M. F. was lower. By increasing the applied E. M. F. from E1to E2 the resultant polarization in the various parts of the earthaffected is increased, resulting in a higher curve for E2 and in ahigher maximum value P2. This will be readily understood when it isremembered that the polarization E. M. F.

arising in any part of the earth cannot be higher than the appliedpotential difference existing in that part of the earth. Butthe'resulting polarization in any part of the earth has a maximum valueabove which it cannot go regardless of the applied E. M. F., and thisresults in the upper surface layers of earth, where the greatestpotential gradient exists, becoming saturated before the lower layers.

This causes equal increments in applied E. M. F. to produce a lessincrease in polarization as the applied E. M. F. is increased. ThusP4-P3 P3P2 P2P1 P1. Another effect of an increased applied E. M. F. isthat the movement of ionswithin the earth is more rapid and the approachtoward equilibrium proceeds more .rapidly. Thus it will be observed thatthe curve E4, for example, bends over at an earlier time than the curveE2 corresponding to a lower value of applied E. M. F.

It will now be clear that as the applied E. M. F. is increased theincrement in polarization. will, due' to the saturation of the upperlayers of earth, be contributed mainly by the under layers of earth.v Asthe applied E. M. F. E progres-.

similar to E1, E2. Ea, E4 may be obtained and the. diflerences betweenthe ordinates of successive curves may be found and plotted to indicatethe polarization characteristics of successively deep- 5 er earthlayers. The method of geophysical prospecting above described,therefore, offers the possibility of determining the characteristics ofdeep underlying strata of earth exclusively.

In practice, in the field, I preferablymake all of the above-describedmeasurements at each.- of a number of different electrode spacings. Thedepth of an earth formation which can contribute to the polarizationeffect is dependent upon the electrode spacing, being greater for agreater electrode spacing. By gradually increasing the electrode spacingand noting at what spacing the curve of Fig. 8 or those of Fig. 14,showing potential differences vs. time, exhibit shapes and valuescharacteristic of certain formations and structures, the depths of theseformations and structures may be estimated. In this manner,

,a zone 34 of oil-bearingTsand, for example, may be located.

y I may determine the maximum potential differ-ence P of one of thecurves of Fig. 14, corresponding to a given applied E. M. F. E1 and agiven polarizing time ab and, keeping E1 and ab constant, vary thedistance between the earthed electrodes. I may then plot the maximumpotential difference obtained for the various electrode spacings andobtain a curve E1 in Fig. 15. I may I then follow the same procedure toobtain a curve E2 for a higher value of applied E. M. F. As theelectrode spacing is increased, deeper lying earth 35 layers become ofimportance in determining the shapes of the curves of Fig. 15. If thereexists in the earth a stratum exhibiting unusually large polarization,the curve E1 in Fig. 15 may start to rise rapidly at the spacing S2, andthe distance S2 together with the E. M. F. Eiwiil serve to inj dicatethe depth of this stratum. ,The rise in curve E2, however, will be'foundto occur at a smaller electrode spacingsi, due to the fact that thelarger applied E. M. F. E2 has been able to aifect this stratum even atthe relatively closer spacing, By this procedure two or more separatebut related sets of data are made available from which to moreaccurately compute the depth of the stratum causing the irregularitiesin the curves.

"While various devices may be employed to act as relays in theperformance of the functions required in making the measurementsdescribed above, I shall describe in detail one such device, 66illustrated in Fig. 12. A distributor 40 hasa large number of segments,here shown for simplicity as only 24 in number, X1, X2, X';,Yi, Y2, Y3,etc., which are successively contacted by the rotating brush 4| whichmoves at a uniform 60 speed in the direction of the arrow. Inconjunction with this distributor 40, a plurality of connectors such asthose generally designated by the numerals 42 and 48 are employed, agroup of segments of the distributor 40 being con-. 05 nected tocontacts of each connector. I have illustrated the distributor 40 ashaving one group of 12 segments X1 to'Xm connected to correspondingcontacts of the X connector 42 and the segments Y1 to Y1: of thedistributor con- 70 nected to corresponding contacts of the Y connector48, similar to connector 42, the segments Y1 to Y12 preferably beingconnected to contacts of the Y connector which correspond in positionrespectively to the contacts of the x 0011- u nector 42 to whichsegments X1 to X1: are connected. In each of the connectors 42 and 48are two plates 43 and 44 independently adjustable through wide anglesabout the common axis 45 and insulated from one another, the plates 43of. both connectors being adapted to be rotated together, as are alsothe plates 44 of both connectors. The plates 43 and 44 are adapted toconnect together varying numbers of the contacts of each connectordepending .upon the angular position of theplates. Contacts 46 and 41are placed one at each end or the row of contacts to make connectionwith the plates 43 and 44 respectively, and the contacts 46 of theconnectors 42 and 48 are connected together, as are the contacts 41 ofboth connectors. It will be observed that with the setting of theconnectors 42 and 48 shown, the arm 4| will be connected to plate 43while passing from segment X1 of the distributor to segment X4, and thatsince segments X5 to X1, inclusive, are connected to contacts of theconnector which do not contact any plate, the arm 4| will be unconnectedduring the time of. passing over these segments. During passage oversegments Xs to X11, the arm 4| is connected to plate 44 and, sincesegment X12 is unconnected, the arm 4| is also unconnected duringpassage by this segment. As the arm 4| passes over the segments Y1 toYl2, the cycle of connection is repeated. Obviously, the distributor maybe constructed with a suificient number of segments to accommodate anydesired number of connectors such as 42 and 48. If, now, the plates 43are connected through contacts 46 to the polarizing circuit containingthe source l9, and the plates 44 are connected through contacts 41 tothe measuring circuit of Fig. 4, it will be seen that the device of Fig.12 can perform the functions required of relay H. The adjustment andconnection of the connectors 42 and 48 as shown in Fig. 12 are such astoresult in a coll dition resembling that of Fig. 6. To get a differentdistribution between the various periods of the cycle, the plates 43 and44 may be rotated to connect more or less contacts and the desirednumber of. segments of the distributor may be left unconnected. To varythe time period of a complete cycle, the speed of rotation of the arm 4|is changed, and to this end I preferably rotate the arm 4| by means of amotor of constant but adjustable speed.

I may, if desired, employ the relay of Fig. 12 in the circuit of Fig.13to connect the polarizing and measuring circuits to four electrodes,as shown, instead of two. The polarizing circuit including the source ofdirect current I9 may be connected to the electrodes 50 and 5| throughthe relay 52, and the measuring circuit comprising instrument 22 andbalancing circuits 23 and 24 may be connected to the separate measuringelectrodes 53 and 54 by the relay 55. A battery 56 energizes either of,the coils of relays 52 and 55 when the appropriate switch of relay IT isclosed. Thus, as relay |'l opens and closes in one or the otherdirection, the relays 52 and 55 act correspondingly to open and closethe polarizing and measuring circuits. In applying the device of Fig. 12as the relay H in this arrangement, the plates 43 and 44 are connectedto the coils of relays 52 and 55 and the arm 4| is connected to thebattery 56.

It is understood that various changes in the method and apparatusdisclosed herein may be made by those skilled in the art withoutdepartduration,

ing from the spirit of the invention as defined in the appended claims.I claim as my invention:

1. A method of geophysical prospecting comprising subjecting the earththrough spaced electrodes to intermittent polarizing, applications of aunidirectional electromotive force of constant duration, intermittentlymeasuring between spaced points in the earth the potential differencedue to the resultant earth polarization during measuring periods ofconstant duration following said polarizing applications by a constanttime interval, and repeating said intermittent measurement of potentialdifference employing measuring periods following said polarizingapplications by time intervals having a plurality of values difierentfrom said first-mentioned time interval, the repetitions of saidmeasurement being suflicient in number to establish a relation betweenthe measured potential difference and said time interval.

2. A method of geophysical prospecting comprising subjecting the earththrough spaced electrodes to intermittent polarizing applications of aunidirectional electromotive force of constant intermittently measuringbetween spaced points in the earth the potential difference due to theresultant earth polarization during measiu'ing periods of constantduration following said polarizing applications by a constant timeinterval, and repeating said intermittent measurement of potentialdifference employing measuring periods of the same duration butfollowing said polarizing applications by time intervals having aplurality of. values diiferent from said first-mentioned time interval,the repetitions of said measurement being sufficient in number toestablish a relation between the measured potential difference and saidtime interval.

3. A method of geophysical prospecting comprising subjecting the earththrough spaced electrodes to intermittent polarizing applications of aunidirectional electromotive force of constant duration, intermittentlymeasuring between spaced points in the earth the potential differencedue to the resultant earth polarization during measuring periods ofconstant duration following said polarizing applications by a constanttime interval, and repeatedly making said intermittent measurement ofpotential difference employing measuring periods of the same durationbut following said polarizing applications by time in tervals having aplurality of values different from said first-mentioned time interval,the times elapsing between the terminations of. the measuring periodsand the beginnings of the polarizing applications remaining the same inall instances.

4. A method of geophysical prospecting comprising subjecting the earththrough spaced electrodes to intermittent polarizing applications of aunidirectional electromotive force, intermittently measuring betweenspaced points in the earth the potential difference due to the resultantearth polarization during measuring periods closely following saidpolarizing applications, and repeating a plurality of times saidintermittent polarization and intermittent measurement, the durations ofsaid polarizing applications being different ineach repetition, and therepetitions of said intermittent polarization and intermittentmeasurement being sufficient in number to establish a relation betweenthe measured potential difference and the duration of the polarizingapplication.

5. A method of geophysical prospecting comprising subjecting the earththrough spaced electrodes to intermittent polarizing applications of aunidirectional electromotive force 01. constant duration, intermittentlymeasuring between spaced points in the earth the potential diil'erencedue to the resultant earth polarization during measuring periods oiconstant duration following said polarizing applications by a constantrelatively short time interval, and repeating said intermittentpolarization and intermittent measurement, said measuring periods beingthe same in each repetition, said time intervals following thepolarizing applications being the same in each repetition, but thedurations of said polarizing applications being diflerent ineach-repetition, and the repetitions of said intermittent polarizationand intermittent measurement being sufiicient in number to establish arelation between the measured potential difference and the duration ofthe polarizing application,

6. A method of geophysical prospecting comprising polarizing the earthfor a plurality of difierent limited time periods by applying aunidirectional electromotlve force thereto through electrodes spacedalong the earths surface in contact therewith, measuring the resultantearth polarization during measuring periods closely i'ollowing theterminations 01 said polarizing periods, again polarizing the earth fora plurality of diflerent limited time periods by applying theretothrough said electrodes a unidirectional electromotive force differentin amount from said first electromotive force, and measuring theresultant earth polarization during measuring periods closely followingthe terminations of said second mentioned polarizing time periods.

7. A method of geophysical prospecting comprising successivelypolarizing the earth by application of unidirectional electromotiveforces 01'. different value to electrodes spaced along the earthssurface in contact therewith, and measuring between spaced points on theearths surface the potential difl'erence due to the resultant earthpolarization corresponding to each of said electromotive forces.

8. A method of geophysical prospecting comprising polarizing the earth aplurality of times by application of a unidirectional electromotiveforce to electrodes spaced along the earths surface in contacttherewith, the amount of said electromotive force being diflerent butthe time of application being the same in each case, and measuring ineach case the potential difference between spaced points on the earthssurface due to the resultant earth polarization.

GLEN PETERSON.

